This invention relates to the fabrication of electric systems of semiconductor circuit chips including cooling devices connected to the chips and, more particularly, to a method of connection of a cooling device to a circuit chip by extrusion of a thermally conductive alloy free of oxide between the chip and the cooling device.
In the construction of electronic systems employing an array of electronic circuit chips interconnected via conductors in a common substrate, it is a common practice to connect thermally some form of cooling device to the circuit chips for removal of heat generated by electric currents flowing within the chips. Good thermal flow from the chips to the cooling devices is important for efficient cooling and maintenance of a desired operating temperature.
One method of interconnection of a cooling device to a chip employs a fusible alloy such as a low temperature solder which is placed between the interfacing surfaces of the cooling device and the chip. Typically, the alloy is prepared as a thin foil which is heated to approximately the liquidus temperature to soften the alloy and to allow extrusion or reflow of the alloy along the interfacing surfaces to form a thermally conductive bonding layer. A thermal bond is attained by micro-conformance of the extruded alloy to the interfacing surfaces.
A problem arises in that imperfections in the metal alloy interface with the chip such as voids, flux residue and oxides decrease the thermal conductivity of the bonding layer. The oxide is of particular concern because it readily develops on the surfaces of the foil or other form of stored alloy material. Upon emplacmeent of the alloy between the interfacing surfaces, there is sufficient oxide to significantly reduce the thermal conductivity of the bonding layer with a corresponding reduction in the efficiency of the chip cooling.